1. Field of the Invention
This invention relates to an improvement in the field of manufacturing advanced composite products intended for aerospace applications and more particularly, but not by way of limitation, to a method of using raw thermoset resin materials and carbon or fiberglass tow to manufacture large composite structures without the use of an oven, an autoclave, or a vacuum bag to cure the resin into a large resin matrix composite structure.
2. Description of the Prior Art
The present space transportation fleet of the United States currently depends on the Space Shuttle and three expendable launch vehicles derived from earlier intercontinental ballistic missiles (Delta, Atlas, and Titan) and their more recent derivatives. The cost to launch payload to low earth orbit by these vehicles varies from $3000 to $5000 per pound. Recent national priorities are attempting to reduce both cost and cycle times to launch by a factor of 10 through programs such as the National Launch System (NLS), and National Aero Space Plane (NASP) as well as Shuttle C (Cargo), Liquid Rocket Booster (LRB), and Hybrid Rocket Booster (HRB). Each of the above programs have ambitious goals in the area of performance, reliability and cost.
Composite materials with their significant property and processing advantages will have a large role to play in meeting the performance, reliability and cost goals of the above programs. This role however must be well chosen and designed as the many advantages of composite materials could easily be overwhelmed by poor application or process selection. An obvious composite application for expendable launch vehicles are payload fairings and various adapter structures. For example, the assignee of the present invention is already applying composites to these types of structures on current Atlas and Centaur launch vehicles.
A more challenging composite application for both expendable reusable launch vehicles is liquid hydrogen and liquid oxygen fuel tanks. Use of composites to contain fluids especially at cryogenic temperatures is currently not well established. However, in both dry and wet applications utilization of low cost material forms and producible fabrication methods are key. It appears that for the kinds of closed structures required for NLS and LRB/HRB vehicles, the wet filament winding process will be featured as the principal method of fabrication and the method of the subject invention would have instant application.
However, certain challenges have to be met for advanced composite product manufacture. First, is the cost. Many graphite/epoxy composites are made in the form of "prepreg" tape or fabric impregnated with resin. Such systems are expensive since the cost per pound for the prepreg system is commonly more the a completed end product made from aluminum. Second, technical issues have contributed to the economic difficulties.
One of the basic challenges in creating large composite parts is to achieve through-the-thickness strength, which requires a secure bond between adjacent plies of fiber. This is necessary to prevent delamination and weakening of the material due to impacts or other damage. The general answer of industry has been to switch to new tough resin matrix systems, but these are more expensive than the older resin types and are more difficult to handle.
Most composite parts are still largely hand-made. At the beginning of the 1980's, many manufacturers believed that automated manufacturing would bring down the costs of composite structures. However, it has been found that automated cutting and tape-laying are possible but require very expensive materials, tooling, and development.
Thus, a true need exists in the manufacturing large composite structures at a reasonable cost, a need which is believed to be met by the instant invention. The primary advantages of the instant invention are the elimination of porosity and the ability to achieve a uniform distribution of resin throughout the composite laminate, regardless of thickness, in an incremental yet continuous manner without the use of prepreg materials or vacuum bag, autoclave processes. Commonly used state-of-the-art methods (e.g., vacuum bagging, multiple staging/compaction operations, autoclave curing, matched die processing, etc.) attempt to achieve a porosity free laminate with uniform resin distribution in discrete (non-continuous) steps. The method of the present invention aligns and automates these discrete operations in a continuous, "assembly-line" fashion, thereby accelerating the entire manufacturing cycle. Furthermore, the present invention is effective for processing/fabricating thick composite laminates, which are particularly susceptible to porosity and poor resin distribution.
The effectiveness of the proposed system in eliminating porosity and achieving resin distribution is based on certain features that are incorporated into the processing method of the invention. Specifically, outgassing of the fiber/resin is accomplished ply-by-ply, and a concentrated, focused source of heat helps to simplify the thermal/polymerization gradients that are established as a result of the polymerization reaction of the resin as curing proceeds. A heated compaction roller/head concentrates pressure that inhibits the entrapment of air at the lay down point and provides adequate force to effect complete laminate consolidation.
The typical approach of processing composite parts utilizing convention prepreg materials uses the standard vacuum bagging/autoclave curing approach. This approach has a number of inherent disadvantages. Considerable effort is expended to create the prepreg material which is, in reality, an intermediate product. Generally, it is manufactured to exacting standards primarily using manual processing methods. The prepreg material is catalyzed and is an activated product, and therefore possesses a finite shelf life, generally 1 to 2 weeks at room temperature for most commercially available forms. To retard further aging, it is then necessary to store the prepreg in a suitable freezer. This factor makes fabrication of large parts difficult and very time-dependent.
Due to the aging of the prepreg material, all subsequent processes in the fabrication sequence must be rigidly controlled (i.e., humidity, temperature, time, etc.). Because the standard prepreg/vacuum bag/autoclave process is a "batch process" where the required plies are laminated together (sometimes amounting to many hundreds of plies) and then cured together, there is extreme difficulty in properly controlling resin content throughout the laminate and eliminating all porosity. In fact, for laminates greater that several square feet in area and more than 0.125 inch in thickness manufacturers utilize intermediate deairing, debulking and compacting steps, all leading to increased cost. The primary cause for the difficulty of processing large and thick laminates in a batch process is due to the anisotropic nature of the resin flow and volatiles outgassing along the fiber direction and between the plies.
Attempts at vacuum curing composite laminates have only been partially successful because even in high vacuum, such as 5.times.10.sup.-4 mm Hg., compression of the laminate has not been great enough to force sufficient removal of the entrapped air from within the laminate. As such, vacuum cured composite laminates tend to be highly porous with mostly unsatisfactory mechanical and physical properties.
While composite parts formed using prepreg-type materials and an autoclave have been satisfactory from a structural standpoint, it has long been desired to eliminate the use of expensive raw materials and equipment such as an autoclave with a method for fabricating composite structures which produces a high quality laminate beginning with the least expensive raw material forms and automated equipment which produces the finished composite part in-situ without the use of vacuum bagging materials or an autoclave. This is basically due to (1) the considerable effort expended to create the prepreg-type material which is in itself only an intermediate product and, generally, it is manufactured to exacting standards thus demanding a high price; (2) the prepreg-type material which is catalyzed and is an activated product, and therefore possesses a finite shelf life, generally less than a year if stored below 0 degrees F. and only 1 to 2 weeks at room temperature; a factor which makes fabrication of large composite structures difficult; (3) the aging of the prepreg-type material, whereby all subsequent processes in the fabrication sequence must be rigidly controlled, e.g., humidity, temperature, out time, to avoid problems during curing; (4) the high cost of autoclaves as compared to conventional ovens; (5) high tooling costs, both initial and maintenance, which would be significantly reduced because of both simplification and a decreased severity of the curing conditions; (6) component rejection rates, which would be minimized due to an elimination of complex curing cycles and the possible loss of vacuum bag integrity during the critical portion of the cure; and (7) production rates, which would be increased due to the elimination of labor intensive vacuum bagging procedures and reliance upon an available autoclave.
Accordingly, it is abundantly clear that the manufacturing methods of the prior art suffer from a number of disadvantages which have inhibited the more widespread application of advanced composite structures. It is believed that many of these shortcomings have been overcome by the present invention.